The members of the cyclin-dependent kinase (CDK) family play critical regulatory roles in proliferation. There are currently 20 known mammalian CDKs. While CDK7-13 have been linked to transcription, only CDK1, 2, 4, and 6 show demonstrable association with the cell cycle. Unique among the mammalian CDKs, CDK7 has consolidated kinase activities, regulating both the cell cycle and transcription. In the cytosol, CDK7 exists as a heterotrimeric complex and is believed to function as a CDK1/2-activating kinase (CAK), whereby phosphorylation of conserved residues in CDK1/2 by CDK7 is required for full catalytic CDK activity and cell cycle progression (Desai et al., “Effects of phosphorylation by CAK on cyclin binding by CDC2 and CDK2.” Mol. Cell Biol. 15, 345-350 (1995); Kaldis et al., “Analysis of CAK activities from human cells.” Eur. J. Biochem. 267, 4213-4221 (2000); Larochelle et al., “Requirements for CDK7 in the assembly of CDK1/cyclin B and activation of CDK2 revealed by chemical genetics in human cells.” Mol. Cell 25, 839-850 (2007)). In the nucleus, CDK7 forms the kinase core of the RNA polymerase (RNAP) II general transcription factor complex and is charged with phosphorylating the C-terminal domain (CTD) of RNAP II, a requisite step in gene transcriptional initiation (Serizawa. et al., “Association of CDK-activating kinase subunits with transcription factor TFIIH.” Nature 374, 280-282 (1995); Shiekhattar et al., “CDK-activating kinase complex is a component of human transcription factor TFIIH.” Nature 374, 283-287 (1995); Drapkin et al., “Human cyclin-dependent kinase-activating kinase exists in three distinct complexes.” Proc. Natl. Acad. Sci. U.S.A 93, 6488-6493 (1996); Liu. et al., “Two cyclin-dependent kinases promote RNA polymerase II transcription and formation of the scaffold complex.” Mol. Cell Biol. 24, 1721-1735 (2004); Akhtar et al., “TFIIH kinase places bivalent marks on the carboxy-terminal domain of RNA polymerase II.” Mol. Cell 34, 387-393 (2009); Glover-Cutter et al., “TFIIH-associated CDK7 kinase functions in phosphorylation of C-terminal domain Ser7 residues, promoter-proximal pausing, and termination by RNA polymerase II.” Mol. Cell Biol. 29, 5455-5464 (2009)). Together, the two functions of CDK7, i.e., CAK and CTD phosphorylation, support critical facets of cellular proliferation, cell cycling, and transcription.
Disruption of RNAP II CTD phosphorylation has been shown to preferentially effect proteins with short half-lives, including those of the anti-apoptotic BCL-2 family (Konig et al., “The novel cyclin-dependent kinase inhibitor flavopiridol downregulates Bcl-2 and induces growth arrest and apoptosis in chronic B-cell leukemia lines.” Blood 1, 4307-4312 (1997); Gojo et al., “The cyclin-dependent kinase inhibitor flavopiridol induces apoptosis in multiple myeloma cells through transcriptional repression and down-regulation of Mcl-1.” Clin. Cancer Res. 8, 3527-3538 (2002)). Cancer cells have demonstrated ability to circumvent pro-cell death signaling through upregulation of BCL-2 family members (Llambi et al., “Apoptosis and oncogenesis: give and take in the BCL-2 family.” Curr. Opin. Genet. Dev. 21, 12-20 (2011)). Therefore, inhibition of human CDK7 kinase activity is likely to result in anti-proliferative activity, and pharmacological inhibition could be used to treat proliferative disorders, including cancer. Indeed, flavopiridol, a non-selective pan-CDK inhibitor that targets CTD kinases, has demonstrated efficacy for the treatment of chronic lymphocytic leukemia (CLL), but suffers from a poor toxicity profile (Lin et al., “Phase II study of flavopiridol in relapsed chronic lymphocytic leukemia demonstrating high response rates in genetically high-risk disease.” J. Clin. Oncol. 27, 6012-6018 (2009); Christian et al., “Flavopiridol in chronic lymphocytic leukemia: a concise review.” Clin. Lymphoma Myeloma 9 Suppl. 3, S179-S185 (2009)). A selective CDK7 inhibitor may hold promise as a therapeutic agent for the treatment of CLL and other cancers.